Why soldiers are ordered to break steps while crossing a bridge?

Resonance: A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object.

Structures like bridges and buildings have a natural frequency of vibration within them.

When the army marches in steps, the frequency of marching coincides with the natural frequency of the bridge. In this case, resonance will occur. If the resonance is strong enough in the bridge and being to vibrate with maximum amplitude. 

Which may cause the destruction of the bridge.

To avoid such a destruction of the bridge soldiers are ordered to break their steps while crossing it. 

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